Field of the Invention
Embodiments of the present invention relate generally to a charged particle beam writing apparatus and a charged particle beam writing method, and more specifically, relate to a method for setting a dose for each pixel in multi-beam writing and raster scan writing, for example.
Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as a unique process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits becomes progressively narrower year by year. The electron beam writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” a mask pattern on a mask blank with electron beams.
As an example employing the electron beam writing technique, a writing apparatus using multi-beams can be cited. Compared with the case of writing a pattern with a single electron beam, since in multi-beam writing it is possible to irradiate multiple beams at a time, the throughput can be greatly increased. For example, in a writing apparatus employing a multi-beam system, multi-beams are formed by letting portions of an electron beam emitted from an electron gun pass through a corresponding hole of a plurality of holes in the mask, blanking control is performed for each beam, and each unblocked beam is reduced by an optical system to decrease a mask image, and deflected by a deflector so as to irradiate a desired position on a target object or “sample”.
For example, in a variable-shaped beam writing apparatus, since a beam of a specific shape can irradiate a desired position, it is possible to perform writing while making the position of a pattern edge and the position of a beam edge correspond to each other. On the other hand, in a multi-beam writing apparatus which cannot arbitrarily control the irradiation position of each beam, a writing target region is divided into a plurality of pixels, and a writing target pattern is converted into pixel patterns (also called bit patterns) which are to be written. Therefore, it is difficult, with respect to all the patterns, to make the positions of a pattern edge and a beam edge correspond to each other. Thus, in a multi-beam writing apparatus, it is desired to adjust a dose of a beam to irradiate a pixel on which the edge of a pattern is located, in order to form the pattern edge at a desired position. Conventionally, as a first method of determining the dose of each pixel, proportioning a beam dose to a pattern area density in a pixel can be cited. As a method similar to the first method, there is disclosed a technique, which is not the case where a beam dose is perfectly in accordance with a pattern area density, but the case where some pixels in an exposure region are exposed to a gray level of 100%, other pixels are exposed to a gray level of 50%, and remaining pixels are exposed to a 0% dose (not exposed at all) (for example, refer to Japanese Patent Application Laid-open (JP-A) No. 2010-123966). As a second method, there can be cited a technique in which if the central point of a pixel is inside a pattern, it is irradiated with a beam dose of 100%, and if a pixel central point is not inside a pattern, it is not irradiated with a beam.
According to the first method, in the case of not performing multiple writing executed while shifting positions, the gradient of a beam dose profile at a pattern edge can be steep, thereby writing in high contrast. However, in the case of performing multiple writing executed while shifting positions, if a pattern, even if only a small part of it, overlaps with a pixel, the pixel is irradiated with a beam, thereby making the gradient of the beam dose profile small and degrading the contrast. Therefore, it becomes difficult to develop the resist in a manner to achieve a highly precise position and critical dimension. According to the second method, when the position of a pixel boundary and the position of a pattern edge do not coincide with each other, since the resolution position of the resist deviates, it is intrinsically difficult to increase the pattern edge accuracy.